Question 751269
x^2+6x+8=0


The term to add and subtract corresponds to {{{(b/2)^2}}} from the expression, {{{x^2+b/a+c/a}}}.  The term for your example would be {{{(6/2)^2=9}}}.


{{{x^2+6x+8+9-9}}}
{{{x^2+6x+9+8-9}}}
{{{(x^2+6x+9)+8-9}}}
{{{(x+3)^2+(-1)}}}
or simply
{{{highlight((x+3)^2-1)}}}